Background: Human immunodeficiency virus (HIV) infection is an epidemic that affects over 1.2 million people in the U.S. Given the lowered immunity of HIV patients, often co-morbidities may posit a public health threat, of particular concern is the coinfection of Clostridium di icile infection (CDI) and methicillin-resistant staphylococcus aureus (MRSA). Despite the empirical evidence highlighting the burden of CDI and MRSA co-infections among HIV patients, there remains a gap in the literature on how such coinfections impact patient and hospital outcomes. As such, the purpose of this study was to utilize the largest national inpatient database in the U.S. to assess patient and healthcare burden of CDI and MRSA among HIV patients.
Methods: This study was a retrospective analysis of the Nationwide Inpatient Sample. The sample included primary HIV discharges, identified using the Clinical Classifications Software Codes while ICD-9-CM codes were used to identify CDI and MRSA. In-hospital mortality was identified from records for reason of discharge as died during hospitalization. Length of stay was provided by NIS and total charges were inflationadjusted using the GDP deflator. Survey weights were used for all analyses.
Results: The prevalence of CDI and MRSA were 2.88% and 3.12%, respectively among HIV discharges. Results demonstrate that HIV patients with CDI and MRSA had a 102% and 43% higher LOS compared with no CDI, respectively. Presence of CDI and MRSA increased in-hospital mortality by 73% and 33%, respectively. Likewise, CDI increased in total charges by 77.6% and MRSA increased in total charges by 52%.

Conclusion: This is the first study to identify the burden of CDI and MRSA infection among HIV patients. Results demonstrate higher healthcare cost and negative patient outcomes resulting from such co-infections. Increased hospital infection control stewardship for such patients is critical in order to improve health equity among HIV patients.

Keywords: HIV, Hospital infections, Health disparities, Healthcare cost

Introduction
Health disparities are prominent among vulnerable populations, especially those with existing chronic or infectious diseases. Human immunodeficiency virus (HIV) infection is an epidemic that currently affects over 1.1 million people in the United States, with majority of new diagnoses among gay/bisexual men, especially of racial/ethnic minority groups [1]. Given the lowered immunity of HIV patients, often co-morbid conditions, especially those common in hospitals may posit a significant public health threat. Of particular, Clostridium difficile infection (CDI), often characterized by diarrhea and colitis [2], is common among hospitalized patients. CDI can be transmitted by fecal microbiota along with several other factors such as frequent hospital exposure, advanced age, gastric acid suppression, and decrease in cellular or humoral immunity [3]. While antibiotics treatment can often be used to treat CDI, such practices posit a significant risk due to the long-term effects on intestinal microbes, which in return reduce the colonization against CDI [4]. In the United States, CDI is the leading cause of hospital-acquired illness (HAI) and contributes to 500,000 cases per year with an estimated 15,000-20,000 annual deaths [5]. For example, CDI increases healthcare costs due to the fact of patients being rehospitalized, extending their hospital stay, medication costs, and laboratory tests. In the United States, an estimated $2,871 to $4,846 per case and $13,655 to $18,067 is attributed for reoccurring CDI cases [6]. CDI poses an urgent threat of infection since it has become highly resistant to antibiotics causing an increase in medical costs and patient deaths [7]. Such infection may further disproportionately impact HIV patients as the literature notes that CDI is the most common bacterial diarrhea among HIV patients [8]. Furthermore, HIV patients hospitalized for CD4 suppression (count < 50cells/mm^3) are at higher risk of contracting CDI when being hospitalized [9]. Patients that are frequently hospitalized are at risk of contracting CDI, but more importantly patients that are living with HIV are at a much higher risk of contracting CDI compared to those that are not immune compromised. For example, Imlay et al (2016), mentioned that targeting antimicrobial infection by focusing on modifiable factors has the opportunity of reducing rates of transmission.

Furthermore, Methicillin-resistant Staphylococcus aureus (MRSA), a multi-resistant nosocomial pathogen [10], posit a significant burden to HIV patients. MRSA frequently occurs in hospitalized patients with compromised immune systems, such as those with HIV, which generates complications such as multidrug resistant strains making it harder to treat the infection [11]. For example, a patient's attributed cost due to MRSA infection in the United States is estimated at $9,275 [12]. Such socioeconomic burden demonstrates the importance of preventing MRSA among patients in the healthcare setting, especially those that are immunocompromised. HIV infected patients continue to experience higher rates of MRSA infections when compared to the general population, often attributable to drug usage or high risk sexual behaviors [13].

Cumulatively, the literature notes the heightened healthcare burden of HAIs, such as CDI and MRSA, and a higher prevalence of such HAIs among HIV patients. However, the current empirical evidence lacks support on the patient and hospital outcome of such co-morbid conditions and thus provides little support for the disproportionate disparity shared among HIV patients. Such an assessment is imperative, as it can provide the true healthcare burden of HAIs among the most vulnerable and scope for infection control stewardship, as well as provide scopes of evidence-based practice in order to improve health equity among such vulnerable populations. As such, the purpose of this study is to utilize the largest national administrative health data in the United States, to assess patient and healthcare burden of CDI and MRSA among HIV patients.

This study was a retrospective analysis of the 2009-2011 Nationwide Inpatient Sample (NIS). For these years, NIS represents a 20% stratified probability sample of community hospitals in the United States that are part of the Healthcare Cost and Utilization Project (HCUP). Further details regarding the NIS can be found elsewhere [14]. While we acknowledge that the limited years may see as a limitation of the study, NIS variables have since changed and does not include several key control variables needed to assess the outcome; thus combining the data would bias the findings. Regardless of the years, our purpose was to assess the burden of CDI and MRSA among HIV patients and thus our results are not dependent on the years.

The study sample included only primary hospital discharges related to HIV infection, identified using the Clinical Classifications Software (CCS) code of 5 for a total sample size of 31,372. International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) codes were used to identify discharges with secondary diagnosis of either CDI (008.45) or MRSA (038.12, 482.42, 041.12). Missing values were excluded from regression analyses.
Patient-level variables included were: age (18-34 years, 35-49 years, 50-64 years, 65 years or more), sex (male, female), race/ethnicity (White, Black, Hispanic, other), payer type (private including HMO, Medicare, Medicaid, other), median household income quartiles based on ZIP code of patient ($1-$38999, $39000-$47999, $48000-$62999, $63000 or more) and Charlson-Deyo Index (6 or less, 7 or more). The Charlson- Deyo index is a validated weighted comorbidity score for each discharge based on ICD-9-CM [15,16].

Hospital-level variables included were: bed size tertile categories (small, medium, large), ownership/control (private investor-own, private non-profit, government nonfederal), setting and teaching status (rural, urban non-teaching, urban teaching), and geographic location (Northeast, Midwest, South, West). Survey year was also included (2009, 2010, 2011).
In-hospital mortality was identified from records for reason of discharge as died during hospitalization versus those who did not. Length of hospital stay was provided from the original NIS variables for each observation. Finally, total charges was assessed from the original variable in the NIS and subsequently inflationadjusted using the Gross Domestic Product (GDP) deflator, with the year 2009 as the reference.

Survey weights were used for all analyses in the study, except when noted. SAS 9.4 (SAS Institute, Inc., Cary, NC) was utilized for linear and logistic regression analyses, while survey-weighted negative binomial regression was conducted using STATA 14 (Stata Corp LP, College Station, TX). Alpha less than 0.05 was set to denote significance.

Secondary CDI and MRSA prevalence as well as descriptive statistics for patient and hospital characteristics were identified from the study population. To assess for associations between CDI or MRSA and patient and hospital characteristics, Chi-square tests using design-based F values were used.
To assess the impact of secondary CDI or MRSA on length of hospital stay, in-hospital mortality, and total charges, negative binomial regression, logistic regression, and linear regression analyses were used, respectively. The natural log transformation of total charges was utilized, and thus the exponentiated coefficients represent percent change in total charges for secondary CDI or MRSA. All regression models were adjusted for patient and hospital characteristics, as well as survey year. This study received appropriate institutional review board approval.

Table 1 displays the study population (primary HIV discharges) characteristics, which included a total of 31,372 HIV discharges. The prevalence of CDI and MRSA were 2.88% and 3.12% among HIV discharges, respectively. Additional study population characteristics listed in the (Table 1). Table 2 shows predictors of secondary CDI and MRSA among primary HIV discharges. Prevalence of CDI was significantly associated with all study population characteristics except for sex, race/ethnicity, and median household income for patient zip code. On the other hand, only insurance type, hospital location/ teaching status, and region of hospital were significantly associated with prevalence of MRSA.

Table 3 further highlights that mean LOS among HIV discharges with CDI was higher than those without (17.96 vs. 8.84 days), as was mean total charges ($113, 459 vs. $58,929). A similar trend of LOS (12.93 vs. 8.97 days) and mean total charges ($96,841 vs. $59,302) were noted for HIV discharges with MRSA. Furthermore, when comparing in-hospital mortality, a higher percent was noted among HIV charges with CDI (12.03% vs. 6.33%) and with MRSA (8.43% vs. 6.43%).
Table 4 demonstrates an impact of secondary CDI and MRSA on LOS, in-hospital mortality, and percent change in total charges among primary HIV discharges based on multivariable regression analyses. After accounting for all control variables, those with CDI had a 102% higher LOS compared with no CDI. Similarly, those with MRSA had a 43% higher LOS compared to those without MRSA. In addition, presence of CDI and MRSA increased inhospital mortality by 73% and 33%, respectively. Likewise, CDI increased in total charges by 77.6% and MRSA increased in total charges by 52%.

The current literature notes that HAIs disproportionately impact immunocompromised patients, such as those with HIV; though limited research exists on the patient and hospital outcomes of such co-morbid conditions. As such, in this study, we utilized the largest inpatient data in the United States to evaluate the patient and hospital outcomes (LOS, in-hospital mortality, total charges) of HIV patients with co-morbid HAIs (CDI and MRSA). Our study's key findings demonstrate that HIV patients with CDI or MRSA reported significantly higher (1) LOS, (2) inhospital mortality, and (3) total charges as compared to those without such HAIs.
While the empirical body of literature has demonstrated that CDI and MRSA are often comorbid among HIV patients, the evidence on the impact of such comorbid conditions on patient and hospital outcomes remains limited. In this study, we noted that mean LOS among HIV discharges was approximately nine and four days longer upon presence of CDI and MRSA, respectively. Previous studies among other vulnerable populations have shown a similar trend. For example, Pant et al. noted that both among renal failure patients and solid organ transplant patients, presence of CDI increased LOS by ten days. In addition, among 193,174 patients that received a liver transplant between 2004 and 2008, 5159 such patient discharges (2.7%) had an an increase of LOS due to CDI [17]. Furthermore, Olanipekun et al. [18] noted a similar trend with patients that were hospitalized due to type 2 diabetes mellitus, which also increased LOS due to CDI.
On the other hand, while studies addressing the risk factors of MRSA are abundant, few have evaluated whether MRSA presence as co-morbid condition increases LOS. Of the few that have, Glance and colleagues noted that patients that were suffering from trauma and/or comorbidities had a higher risk of acquiring MRSA, which in turn increased the patients, LOS [19].
Similarly, in-hospital mortality has been shown to increase due to presence of HAIs, especially CDI in the literature; similar to the trend we noted among HIV discharges in our study. For example, patients with type 2 diabetes mellitus had over 260% increased odds in-hospital mortality with CDI as compared to those without CDI [18]. Likewise, liver transplant patients with CDI had higher prevalence of in-hospital mortality as compared to those without CDI (5.5% vs. 3.2%) as well as 70% higher odds of such outcome in the presence of CDI [17]. Pant, et al. [20] also identified 184,139 cases of patients suffering from end-stage of renal disease, of whom 2.8% had CDI. Among such patients, presence of CDI increased rate of in-hospital mortality (13.2% vs. 5.3%). Furthermore, Olanipekun and colleagues [18] identified that patients with HAIs had significantly higher inhospital mortality as compared to those without HAIs. For example, patients with CDI had a 1.5-to1.9-fold higher odds of in-hospitality mortality when compared to the controls. These findings, in addition to our results, demonstrate the increased burden of HAIs on patient outcomes, with our results expanding the body of literature on such a burden among HIV patients.
Undoubtedly, such increased rates of LOS and in-hospital mortality further contribute to the increased health care cost. For example, average hospital cost among patients with type 2 diabetes mellitus is $23,000 with CDI as a comorbidity, as compared to $9,100 without CDI comorbidity. A similar trend is noted among liver transplant patients ($143,000 vs. $73,000) and end-stage renal disease patients $124,846 vs. $56,663, where presence of CDI substantially increased health care cost (17,20).
Likewise, Glance et al. [19] found increase in medical cost between 2.6 to 6 times higher upon presence of MRSA. In support of this, when comparing 390 patients infected with methicillinsusceptible Staphylococcus aureus (MSSA) to 335 patients infected with MRSA, Filice, et al. [21] reported that for patients with MRSA had significantly higher cost as compared to those MSSA only, thus demonstrating the increased healthcare burden of MRSA. One major limitation in the field remains to be evaluation of MRSA impact as a HAI on chronically ill patients. Much of the literature we reviewed evaluated the burden of MRSA alone, but little on prevalence among other patient populations. Thus, our study results expand this very limited body of literature on the prevalence of MRSA among HIV patients as well as the outcomes associated with such co-infection. Cumulatively, the literature and our study results demonstrate that HAIs, such as CDI and MRSA, substantially impact patient and hospital outcomes among HIV patients; further providing several clinical care and research implications.
The literature notes that HIV patients are more likely to be admitted at a higher frequency to hospitals and this in turn can increase their risk of contracting HAIs due to their immunocompromised state. As such, providers working with HIV patients must minimize invasive procedures, such as, invasive lines or mechanical ventilation when treating for a short duration of time. In addition, if a HIV patient is going to undergo an invasive ventilation procedure, sterility is crucial to reduce the chances of HAI's, which in return would increase the cost of medical expenses [22].

Furthermore, CDI is contracted from contaminated surfaces or by direct hand contact from a healthcare provider and thus, increased vigilance in appropriate isolation and/ or de-contamination methods are needed when treating HIV patients [23]. Based on the findings of existing literature, chlorine-releasing agents are more effective than detergents for killing spores produced by CDI, however, the best strategy in reducing CDI transmission is handwashing since alcohol gels are not as effective [24].

In addition, MRSA's most common mode of transmission is among providers treating patients through direct contact during a procedure. Prevention of HAIs is also a major national priority as outlined by the Patient Protection and Affordable Care Act that established Hospital-Acquired Condition (HAC)-Reduction Program, which provides incentives for hospitals to reduce such cases while also mandating payment adjustments for hospital that fail to perform well [25].

The results of this study should be interpreted in the context of its limitations. We limited our results to 2009-2011 NIS data due to changes in NIS data reporting and collection style and thus to limit any misclassification biases. Furthermore, due to lack of patient identifiers, we cannot assess readmission rates among the target population. In addition, other social determinants of health, beyond those included in NIS, could not be assessed. For example, in a systematic evaluation, O'Brien et al. [26] noted that aerobic and/or resistive exercise can improve quality of life and outcomes among HIV patients.
Similarly, Weiser and group [27] found higher mortality among food insecure HIV patients, as compared to their food securecounterparts. Whether such factors attenuate the outcomes noted in this study remains to be evaluated in future studies. Furthermore, CDI and MRSA infections could also be contracted outside of the hospital and thus the results shown here may not capture the full burden.

Not withstanding such limitations, our study has several strengths and thus contributions to the field. Unlike majority of studies that are limited to specific hospitals, our study utilizes nationally representative data, and thus the increased external validity allows for generalizability of the results. Likewise, NIS data includes the uninsured, public, and private payers, unlike other databases, such as Medicare claims. Furthermore, while a significant amount of literature has highlighted the prevalence of HAIs among HIV patients, our study adds to the body of literature by providing a comprehensive assessment using the largest inpatient database in the United States on the patient and hospital outcomes associated with such comorbidities. Similarly, while a plethora of studies evaluated the prevalence of MRSA as a HAI, the literature remains limited on the burden among chronically ill patients. In fact, a major barrier we noted in our study is to find comparable studies focused on MRSA, and thus our results add to the very limited body of literature on the burden of MRSA as a comorbid condition.

CDI: Clostridium difficile infection
HAI: Hospital-acquired infection
HIV: Human immunodeficiency virus
LOS: Length of stay
MRSA: Methicillin-resistant Staphylococcus aureus
NIS: National (Nationwide) Inpatient Sample Declarations
-Ethics approval and consent to participate: This study was approved by the California State University, San Bernardino Institutional Review Board.

- Consent to publish: Yes
-Availability of data and materials: The data is publically available from AHRQ: https://www.hcup-us.ahrq.gov/db/nation/nis/nisdbdocumentation.jsp
-Competing interests: None to declare.
-Funding: Mr. Robert Avina was funded by CSUSB's student research and travel grants. Dr. Monideepa Becerra was funded by faculty professional development grant.
-Authors' Contributions: Dr. Monideepa Becerra and Dr. Benjamin Becerra are co-PIs of the study. Dr. Monideepa Becerra was involved in concept development, data interpretation and final manuscript development. Dr. Benjamin Becerra was responsible for data analysis and manuscript approval. Mr. Robert Avina was primarily responsible for data interpretation and manuscript draft development. Ms. Karina Corral was responsible for data interpretation and help in manuscript development.

Funding source
RMA was funded by student research grant at California State University, San Bernardino.
MBB was funded through faculty professional development grant at California State University, San Bernardino.

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